Hi,just thought i'd say that i have trouble levelling my seatbox sometimes as i'm new to seatboxes.So i went to B and Q and bought a cheap plastic level for £1.98,cut it up with a hacksaw and stuck it to my seatbox using double sided foam tape that my dad uses for his model planes.It's not deadly accurate but puts you in the right area.

Make sure you level the box first with a proper spirit level,then place the two new levels where they are most accurate to the original level.Job done,i hope this hasn't already been done,